The subject matter of the present invention relates to the subject matter of the copending patent application entitled "Improved Cache Memory Utilizing Selective Clearing and Least Recently Used Updating", Ser. No. 114,854, filed on Jan. 24, 1980 assigned to the assignee of the present invention. That Application discloses a cache memory system which utilizes a least recently used updating subsystem in the cache control for improving the speed of the access by that device. The purpose of the least recently used subsystem is to record the order of usage of the level at a selected column address and the primary directory and to update this information after each transaction with the cache. By keeping a constant record of which level of the selected column of the primary directory is least recently used, it is possible to displace the least recently used level upon a cache miss. By updating the least recently used level or an empty level of the primary directory and the corresponding location in cache store as opposed to a level determined by a round robin counter, it is possible to decrease the access time for transactions with the cache because of the probability that data recently used by a program will be used again in the near future.
The least recently used subsystem has separate utility in any application where the order of usage of certain devices or memory locations is important. Such applications might include control systems, priority networks and in many applications either currently in use or still to be conceived.